1. Field of the Invention
The present invention relates to a multimode surface acoustic wave filter, especially a transverse-coupling type multimode filter. More particularly, the present invention relates to a transverse-coupling type multimode filter wherein surface acoustic wave resonators utilizing an IDT (Interdigital Transducer) are arranged in a direction perpendicular to the propagation direction of the surface acoustic wave.
2. Description of the Related Art
Recently, surface acoustic wave filters are used in mobile communication terminals such as portable telephones because they have a small size and low device height, and can achieve sharp filter characteristics.
Among the surface acoustic wave filters, the transverse-coupling type multimode filters have characteristics suitable as narrow bandpass filters. Especially, use of quartz substrates has realized narrow bandpass filters which are excellent in temperature stability and with a relative bandwidth of about 0.05%.
Moreover, transverse-coupling type multimode filters are used as an IF (Intermediate Frequency) filter for portable telephones because they involve a small occurrence of spurious response outside the passband.
However, mobile communication terminals including digital cordless telephones such as a PHS which have recently come into service require a filter having a relative bandwidth of about 0.1% and good temperature stabilty in accordance with the digitization of the system.
In order to realize a filter having a relative bandwidth of about 0.1%, there are two conventional methods such as the following.
One method is to use a piezoelectric substrate having good temperature stabilty and a large electromechanical coupling coefficient. The other method is to employ a complicated filter construction other than a transverse-coupling type multimode filter by using a quartz substrate. An example thereof is a composite longitudinal mode resonator filter such as disclosed in Japanese Unexamined (Kokai) Patent Publication No. Hei 6(1994)-232687.
The only substrate material that is now available and that has good temperature stabilty and a large electromechanical coupling coefficient attaining the above-mentioned design as required is one made of Li.sub.2 B.sub.4 O.sub.7.
However, Li.sub.2 B.sub.4 O.sub.7 has a larger temperature coefficient of second order relative to the surface acoustic wave than a quartz substrate and is prone to corrosion by water, rendering it difficult to manufacture the filters.
The composite longitudinal mode resonator filters as disclosed in the above-mentioned Japanese Unexamined Patent Publication No. Hei 6(1994)-232687 are constructed to include two IDTs and reflectors on both sides thereof, so that the filter size is large and unnecessary spurious responses outside the passband are generated in a considerably large amount, thereby causing noises.